A crucial concept to understand in jiu jitsu and submission grappling is that of tightness. Once a lock is put in place, it is crucial that there be sufficient tightness that an opponent cannot easily pull out (unless you are deliberately employing a tactic to allow him out to set up another move). The problem is that most people always use MUSCULAR tightness as the means of tightening a lock. Muscular tightness is tiring to maintain and usually inhibits movement. A much better sense of tightness to aspire to is MECHANICAL tightness, where my primary focus is upon a tight fit of my body into that of my opponent. In this way, BODY PLACEMENT AND POSITIONING create tightness around a lock rather than muscular exertion. The only degree of muscular exertion i need is that required to hold my body as a system of WEDGES AROUND THE PART OF MY OPPONENTS BODY THAT I AM TRYING TO IMMOBILIZE AND CONTROL. Here, Garry Tonon shows good mechanical tightness as he enters a juji gatame arm lock from bottom position. By using only enough muscular exertion to maintain his body position, he can maintain the lock for long periods of time without exhaustion and if necessary, move smoothly in response to his opponents defense. Learning to differentiate muscular tightness from mechanical tightness is one of the biggest conceptual steps students can make in their path to excellence.